1. Field of the Invention
This invention relates to drilling in earth formations located beneath a body of water, such as in the Gulf of Mexico, and in which the drilling operations are conducted from a floating vessel. More specifically, this invention relates to a method and apparatus involving a riser pipe which extends from the ocean floor to a floating vessel located at the surface of the body of water. It relates especially to a novel riser pipe in which there are means to change the effective size of the riser and reduce the amount of tension needed to be applied to the riser pipe.
2. Setting of the Invention
In recent years, it has become desirable to use a floating vessel from which to drill wells in a marine location. In such operations, the floating vessel is sometimes connected to a submerged wellbore by a long tubular member through which drilling tools, drilling fluids, etc., pass between the vessel and the wellbore. This long tubular member is commonly referred to as a riser pipe. The vessel is maintained on location normally by long cables which are connected to anchors in the ocean floor. Alternatively, dynamic positioning units can be provided from the vessel.
The submerged wellhead usually includes a blowout preventer and other control equipment. In one embodiment, the upper part of the wellhead assembly includes a ball connector which provides a flexible connection between the wellhead assembly and the riser pipe. The lower end of the riser pipe is connected to this ball joint and is free to pivot thereabout. This is commonly called a flex joint. Other types of flex joints are commercially available; however the ball and socket joint is enjoying high popularity. Although a vessel is anchored, it can have vertical movement of from a few feet up to 25-30 feet or more. To compensate for this vertical movement, a slip or telescopic joint is provided in the riser pipe. The slip joint is usually located at the top of the riser pipe to avoid the need for high pressure seals and so that it can be serviced more easily than if it were placed on the bottom.
If the riser pipe is supported solely at its lower end, its only effective weight, i.e., weight in water, causes it to be in a state of axial compression increasing from 0 at the top to a maximum at the bottom support. A drilling fluid is normally circulated down the drill string within the riser pipe and back up to the surface in the space between the riser pipe and external wall of the drill string. The weight of this drilling fluid may vary from about 81/2 lbs/gal up to about 13, 15 or more lbs/gal. The weight of the drilling mud also has a degrading effect on column stability (buckling) of the riser pipe. To counteract this buckling effect, it has become a practice to apply a tensile force to the top of the riser pipe. Special tensioning devices are mounted on the ship and have their cables attached to the upper end of the riser pipe but below the slip joint. These tensioning devices are commonly referred to as constant tensioning devices so that they can maintain a constant tension on the riser pipe, although the ship may rise and fall with respect to the riser pipe. These constant tensioning systems are helpful but are costly and must be maintained, maintenance requirements being proportionate to tension output. In order to reduce the tension which must be applied to the top of the riser pipe, it has been suggested that a flotation assembly be added to the exterior of the riser pipe. For example, some assemblies include a premolded foam core that is bonded directly to the outer wall of the pipe and is completely exposed about its outer surface to the water in which it is immersed.
2. Prior Art
The closest prior art of which we are aware is U.S. Pat. No. 3,729,756, issued May 1, 1972, entitled "Flotation Assembly". This patent describes a collar to be placed around the riser pipe and comprises a pair of semiannular flotation members, each of which includes a semicircle outer shell of fiberglass, a semiannular low density core preferably including a plurality of plastic hollow spheres surrounded by synthetic foam, and arcuately shaped clamping means imbedded in the core. We know of no art which teaches to modify the interior of the riser pipe to reduce the required tension in the manner as taught herein.